Aptana is a robust, JavaScript-focused IDE for building dynamic web applications. Highlights include the following features:
Code Assist on JavaScript, HTML, and CSS languages, including your own JavaScript functions
Outliner that gives a snapshot view of your JavaScript, HTML, and CSS code structure
Error and warning notification for your code
Support for Aptana UI customization and extensions
Cross-platform support
Free and open source. (Source available soon)

this is the best,
I have used dreamweaver cs3 as well as this one and I think this app together with cssedit is the best combination to set up your site.
dreamweaver takes up way to many resources even on my brand new macbook pro with 2GB of internal mem.
this just runs like a breeze.